Exfoliated carbon nitride as solar sensitive materials for photocatalytic applications

Exfoliated carbon nitride as solar sensitive materials for photocatalytic applications Simge Naz Degerli 1, Matteo Tommasi 1, Gianguido Ramis 2, and Ilenia Rossetti 1* 1 Chemical Plants and Industrial Chemistry Group, Dip. Chimica, Università degli Studi di Milano, CNR-ISTM and INSTM Unit Milano-Università, via C. Golgi 19, 20133 Milan, Italy, ilenia.rossetti@unimi.it, presenting author * corresponding author 2 Dip. Ing. Chimica, Civile ed Ambientale, Università degli Studi di Genova and INSTM Unit Genova, via all’Opera Pia 15A, 16145 Genoa, Italy, gianguidoramis@unige.it INTRODUCTION The metal-free catalyst, graphitic carbon nitride (g-C3N4) is an interesting new material with spreading interest for photocatalytic applications. It can be produced from C and N containing precursors, usually inexpensive and it arranges as a bulk material constituted of assemblies of graphite-like 2D layers. It shows a narrow band gap (ca. 2.7 eV) allowing to exploit visible light. In order to improve its activity for photocatalytic applications, different strategies can be followed. In particular, exfoliation leads to a delaminated material which offers higher surface area (from ca. 10 m2/g of the bulk material to 40 m2/g) and limited charge recombination. The aim of this work is to synthesis nanophotocatalysts based on g-C3N4, as such or loaded over TiO2 or WO3, for different photocatalytic applications, in particular the photoreduction of CO2 at high pressure (up to 20 bar), the photodegradation of pollutants from wastewater and the production of green hydrogen by photoreforming of carbohydrates. EXPERIMENTAL/THEORETICAL STUDY Bulk graphitic carbon nitride (g-C3N4) was prepared by thermal condensation of different precursors, melamine, dicyanamide, dicyandiamide, urea and thiourea. The precursor was placed in a covered ceramic crucible and heated at 2 °C min-1 up to 500-600 °C. The resulting yellow powder underwent a successive exfoliation treatment by using ultrasounds with a tuneable US probe able to impart different energy for different time and with selectable pulses to the material. Binary materials composed of C3N4 and TiO2 P25 or WO3 were also prepared by mechanically mixing both components in different proportions. All the tests for the photoreduction of CO2 and for the photoreforming of carbohydrates were performed using an innovative pressurized batch photo-reactor, designed to work under pressure up to 20 bar and temperatures not higher than 90 °C. Irradiation is accomplished by means of a 125 W medium pressure Hg vapour lamp made of two bulbs which emits in the range of 254-364 nm (average irradiance 152.63 W/m2). The optimal catalyst and HS concentration were 31 mg·L-1 of photocatalyst and 1.67 g·L-1 of hole scavenger (Na2SO3). Each test lasted for 3-24 h. The products in liquid phase were analysed via HPLC (LC-4000 series, Jasco) and the gas phase products by GC (Agilent 7890). Photodegradation of Acid Orange7 (AO7) dye was accomplished with either 125 W medium pressure Hg vapour lamp and with a visible LED lamp. RESULTS AND DISCUSSION Materials characterization evidenced that the band gap of all the g-C3N4 materials was compatible with the use of blue light to photoexcite the catalyst. Interestingly, delamination increased slightly the band gap, but it also allowed to increase the lifetime of the photogenerated charges, as determined by spectrofluorimetry. The best precursor for the preparation of the materials were dicyanamide and melamine and the increase of the power of the US during delamination or the time of treatment was overall beneficial. Production of formic acid resulted significant (tens of mol(h kgcat) and as well the production of H2 by photoreforming of glucose. The best catalysts allowed full degradation of AO7 within 100 min using a 125 W UV lamp, whie the performance under visible white light was much lower. ACKNOWLEDGMENTS The authors are grateful to Fondazione Cariplo (Italy) for supporting this research through the grant 2021-0855 - “SCORE - Solar Energy for Circular CO2 Photoconversion and Chemicals Regeneration” within the Circular Economy call 2021.